Optical MEMs devices are of considerable importance in optical communication systems. In one important application, a two-dimensional array of MEMs devices provides an optical cross connect between input optical paths and output optical paths. Each MEMs device in the array is a movable mirror disposed to receive an input optical signal. The mirror can be electromechanically moved to reflect the received optical input to a desired output path.
A typical MEMs mirror comprises a metal-coated base movably coupled to a surrounding frame via a gimbal. Two torsional members on opposite sides of the mirror connect the mirror to the gimbal and define the mirror's axis of rotation. The gimbal, in turn, is coupled to the surrounding silicon frame via two torsional members, defining a second axis of rotation orthogonal to that of the mirror. A light beam can therefore be steered in almost any direction.
Electromechanical actuators, such as electrodes, are disposed in a cavity underlying the mirror and the gimbal. Voltages applied to electrode actuators control the orientation of the mirror and the gimbal. Alternatively, in modified arrangements, electrical signals control magnetic, piezoelectric or thermal actuators.
The mirrors used in optical MEMs devices are typically small substrates coated with metal that is highly reflective at the wavelength of interest. MEMs mirrors typically have a maximum dimension (usually a diameter) of less than 3000 micrometers. Metal coatings such as gold, silver, rhodium, platinum, copper and aluminum can be used to reflect communications wavelengths in the range 800-1600 nm. These reflective metal films typically have thicknesses ranging from about 20 nm to about 2000 nm. The films are deposited on a movable base, usually silicon. At least one adhesion-promoting bond layer is optionally added between the reflective metal film and the base in order to prevent the reflective metal film from peeling.
A difficulty with such devices, especially where multiple reflections are needed, is loss of optical signal. The mirror surfaces are typically planar. Consequently the reflected beams are not focused. Lens arrays can be added, but their addition presents problems of loss, alignment and expense. Accordingly there is a need for MEMs devices with mirrors that are more efficient in directing and focusing light.